Intensification screen for radiographic film

ABSTRACT

A layer comprising particles of a low melting point metal alloy of lead, bismuth, tin and cadmium and a binder is provided as an intensification screen. Good intensification without fog is obtained using particles of alloys, such as Wood metal, which melt lower than 80* C.

United States Patent Inventor Raymond Marius Augustin Bayol Vincenncs,France Appl..No, 724,256

Filed Apr. 25, 1968 Patented Aug. 3, 1971 Assignee Eastman Kodak CompanyRochester, N.Y.

Priority Feb. 22, 1968 France 140896 lN'IENSll-ICATION SCREEN FORRADIOGRAPHIC FILM [50] Field of Search 250/65; 96/82 [56] ReferencesCited UNITED STATES PATENTS 2,739,243 1/1953 Sheldon 250/65 2,906,88111/1958 Mittelstaedt 250/65 2,950,972 8/1960 Mueller 250/65 PrimaryExaminer-Norman G. Torchin Assistant Examiner-John L. GoodrowAttorneys-W. H. J. Kline, Bernard D. Wiese and Gordon L.

Hart 1 ABSTRACT: A layer comprising particles of a low melting pointmetal alloy of lead, bismuth, tin and cadmium and a binder is providedas an intensification screen. Good intensification without fog isobtained using particles of alloys, such as Wood metal, which melt lowerthan 80C.

HNTENSHFHCATHON SCREEN FOR lRADllOGlRAPlHlC FILM The present inventionrelates to intensification screens for radiographic films. Moreparticularly, the invention relates to coated layers as intensificationscreens for radiographic films and to radiographic films including suchlayers, and i to a method for making such layers.

Radiation such as X and y rays, which include much more energy thanlight rays, have a powerful effect on photographic layers when the X and'y rays are absorbed. However, only a very small proportion of X or 7rays are absorbed by the photographic layer and for this reason, thelayers are generally richer in silver halide and thicker thanlight-sensitive photographic layers. To obtain thick layers, an emulsionis usually provided on both sides ofa radiographic film base since asingle thick layer could not be conveniently developed and fixed. it haslong been known to markedly increase the effective sensitivity ofradiographic film by providing an intensification screen adjacent to thesilver halide emulsion. Upon irradiation, an intensifying screen of thetype we are dealing with absorbs high energy X-rays which causessecondary electron emission to which the emulsion is highly sensitive.Variousintensification screens of this type are known, employing heavymetals such as lead, usually in the form ofa metal plate or foil.

It is an object of the present invention to provide a newintensification screen for radiographic films which greatly increasesthe effective X and y rays sensitivity of the film without causingemulsion fogging. This and other objects are achieved according to theinvention by providing an intensification screen for radiography and thelike comprising a layer with particles of a low melting point alloywhich comprises a heavy metal that emits secondary electrons whenexposed to X or y radiation, dispersed in a film-forming binder.

A preferred alloy has a low melting point, below 95 C. and preferablyless than 80 C. The major ingredient is bismuth, which generallyconstitutes about one-half of the weight ofthe alloy. Lead makes upabout one-fourth and cadmium and tin about one-eighth each. Wood alloy,having the composition Pb 25 percent, Bi 50 percent, Sn 12.5 percent andCd 12.5 percent, and melting point of 70 C. is most preferred, but theconstitution of each of the ingredients can be varied provided themelting point of the alloy remains low. The melting point.

should not exceed the boiling point of water and, for that reason,melting points of less than about 95C. are preferred since a pressurizedprocess would berequired to reduce water losses to a controllabledegree. Still more preferably, the melting point is below 80C.

The alloy is incorporated in an intensification screen layer by simplyheating a mixture of the alloy and a suitable binder to melt the alloy,and dispersing the molten alloy in the binder and forming the dispersioninto a layer or film. The layer can be self-supporting and applied assuch to a radiographic film or it can be coated on a support which isapplied to a radio graphic film or it can be coated directly on aradiographic film all as will become more apparent in light of thefollowing detailed description which includes a preferred embodiment ofthe invention. 7

A radiographic film for use withan intensification screen according tothe invention can be any film plate or the like containing-one-or moresilver halide or other radiation sensb tive layers provided on asupport. The silver content of such radiation-sensitive silver halideemulsions is generally higher than for other light-sensitive emulsions,reaching up to 3.6 grams per square foot. The iodide content isrelatively high and the binder content is generally low. Gelatin is apreferred binder and hardened gelatin is preferred. It isquite'important that the film be resistant to chemical fog.

This invention can be further illustrated by the following example of apreferred embodiment thereof although it will be understood that thisexample is included merelyfor purposes of illustration and is notintended to limit the scope of the in vention unless otherwisespecifically indicated.

EXAMPLE An intensifying screen composition is prepared by heating amixture of 300 grams of Wood alloy and 30milliliters of dibutylphthalate, which is used to stabilize the final dispersion, to C. tomelt the alloy. The molten alloy is then added to 200 grams of a l0percent gelatin solution maintained at a temperature of 80 C. andvigorously mixed for 2 minutes to disperse the molten alloy at thattemperature. The mixture is then cooled, with continued stirring, belowthe melting point of the alloy to 40 C. which causes the alloy to formsmall solid spherical particles having a diameter ofabout 6 to 7microns.

The resultant dispersion is coated. directly on a polyester(polyethylene terephthalate) support 0.06-millimeters thick at acoverage of 3 grams alloy per square decimeter. The gelatin binder setson cooling. Examination of the dried layers shows that the alloy beadshave settled by gravity and are concentrated mostly at the polyestersupport interface leaving mostly gelatin binder at the outer surface.This settling effect is helpful to improve resolution by bringing thealloy-rich side of the layer into contact with the silver emulsion.

In various embodiments this can be accomplished in several ways. Thecoated layer can be wet-transferred from a temporary support on which ithas been coated to a permanent receiving supporting sheet, with thealloy-rich side outward. For an integral support the layer can be coatedand settled directly on the surface ofa radiographic recording element,or a layer can be wet-transferred from a temporary coating support, onwhich the layer has been coated and settled, to the surface having asubstratum, after stripping off the temporary support, the resultingmaterial is applied onto the radiographic element, with the alloy-richface inward against the recording element.

in some embodiments, if the settling effect is not preferred,sedimentation of alloy particles can be inhibited by coating a moreviscous dispersion or by rapidly settling the coated layer, as by rapidcooling, or both.

Two screens are made: One by the settling technique described, followedby wet-transfer to a permanent support of 0.06-millimeter polyester filmbase, alloy side out; the second by coating the dispersion at a lowertemperature and with rapid cooling to quickly set the gel and minimizesettling. Both coatings contain 3 grams of alloy per square decimeterand 0.06-millimeter polyester film base. The screens are tested with aconventional silver halide radiographic film with exposure to X-rays at240 KV, 3 ma. using a 6-millimeter copper filter, conventionalprocessing. A control test is run with a conventional lead screen.-Results are tabulated showing density of the developed films afterexposure with the respective screens.

Wood Alloy Screen (concentrated at Wood Alloy Screen Lead Screen(dispersed through (Control) outer surface) layer) 2.03 "1.9a 1.44 2.04L89 L46 2.04 L89 1.46

2.04 Average D varying the temperature, agitation, binder content andnature and in several other ways as will be apparent to one of ordinaryskill in the art. Generally speaking, however, the average particle sizeis less than microns and in the most preferred embodiments between I and12 microns.

The nature of the binder of the screen can vary. Unhardened gelatin is apreferred binder, but others, including self-supporting resin sheets canbe used depending primarily upon the manner in which the screen isutilized. Where the screen is provided in a layer distinct from theradiographic film and not integral therewith, the alloy can be providedin virtually any settable binder such as a thermoplastic natural orsynthetic resin, gum or gel. The screen is merely placed next to theradiographic silver halide emulsion prior to exposure. In the event thatlong contact between some particular binder and the silver halideemulsion would cause some difficulty such as the formation of fog, thescreen can be kept away until immediately before exposure. lnembodiments where the screen is provided in a layer integral with thefilm, it will usually be preferably that the screen be removed prior todeveloping and this can be accomplished by providing a screen layerwhich can be removable in any of several ways. The layer, for example,can be melted and/or dissolved off, such as be treating in hot water.Alternatively, the layer can be applied over a water-soluble adhesivelayer. Gelatin, polyvinyl alcohol, polyvinyl pyrrolidone and the likeare watersoftenable and are suitable for such uses as binders andadhesive sublayers.

The relative concentrations of alloy and binder in the intensificationscreen can vary quite widely. The layer is preferably richer in thealloy but can contain low amounts ifa low degree of intensification isdesired. Generally, however, the layer will contain at least percent byweight of the alloy and up to 95 percent or more. The function of thescreen is to provide the particles of intensifying material close to thesilver halide emulsion during exposure thereofto X-rays and the amountof alloy in the layer is conveniently defined in terms of the amount ofalloy provided per unit area since, when in operation, each unit area ofthe screen will'correspond to the same size unit area of the silverhalide layer. The amount of alloy calculated in this manner willgenerally be between 0.l and I0 grams per square decimeter.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention as described hereinabove and as defined in the appendedclaims.

I claim:

1. In a method of recording a radiographic image on a silver halideemulsion during exposure thereof to X-ray radiation in the presence ofan adjacent intensification screen, the improvement comprising employingin said screen an alloy, having a melting point below C. of lead,bismuth, tin and cadmium.

2. A radiographic film comprising a support, at least onelight-sensitive silver halide emulsion layer, and an intensificationscreen layer above said silver halide emulsion layer, saidintensification screen layer comprising a binder and particles of analloy of lead, bismuth, tin and cadmium having a melting point oflessthan 95C.

3. A radiographic film according to claim 2 wherein said intensificationlayer is removable from said film.

4. A radiographic film according to claim 2 wherein said intensificationlayer comprises a water-softenable binder.

S. A radiographic film according to claim 2 wherein said intensificationlayer is secured to said emulsion layer by a watersoluble adhesivelayer.

6. A radiographic film according to claim 2 wherein said intensificationscreen layer comprises from about 25 to about 95 percent by weight ofsaid alloy particles based on the weight of the layer.

7. A radiographic film according to claim 6 wherein said alloy particleshave an average particle size of from 1 to 12 microns.

8. A radiographic film according to claim 7 wherein the binder of saidintensification layer comprises gelatin,

9. A radiographic film according to claim 8 wherein the alloy isprovided in the intensification layer in an amount of from 0.! to 10grams per square decimeter.

10. A radiographic film according to claim 9 wherein said alloy has thefollowing composition:

Lead 25 weight percent Bismuth 50 weight percent Tin l2.5 weight percentCadmium IILS welghl percent

2. A radiographic film comprising a support, at least onelight-sensitive silver halide emulsion layer, and an intensificationscreen layer above said silver halide emulsion layer, saidintensification screen layer comprising a binder and particles of analloy of lead, bismuth, tin and cadmium having a melting point of lessthan 95*C.
 3. A radiographic film according to claim 2 wherein saidintensification layer is removable from said film.
 4. A radiographicfilm according to claim 2 wherein said intensification layer comprises awater-softenable binder.
 5. A radiographic film according to claim 2wherein said intensification layer is secured to said emulsion layer bya water-soluble adhesive layer.
 6. A radiographic film according toclaim 2 wherein said intensification screen layer comprises from about25 to about 95 percent by weight of said alloy particles based on theweight of the layer.
 7. A radiographic film according to claim 6 whereinsaid alloy particles have an average particle size of from 1 to 12microns.
 8. A radiographic film according to claim 7 wherein the binderof said intensification layer comprises gelatin.
 9. A radiographic filmaccording to claim 8 wherein the alloy is provided in theintensification layer in an amount of from 0.1 to 10 grams per squaredecimeter.
 10. A radiographic film according to claim 9 wherein saidalloy has the following composition: Lead25 weight percentBismuth50weight percentTin12.5 weight percentCadmium12.5 weight percent.